I’ve been asked to write about Dark Energy. Describing Dark Energy in a 500-word column is like describing a symphony by humming the first note. But here goes.

The story begins with Dark Matter. Four hundred years ago, Isaac Newton described the motion of heavenly bodies based on gravity. In 1933, the Bulgarian-born astronomer Fritz Zwicky, working in the U.S., noticed unexpected motion in galaxies. Zwicky inferred gravitational effects of unseen material which he called Dark Matter. Scientists don’t believe it’s simply something “out of sight”, but something inherently invisible. However, they’ve been unable actually to find Dark Matter. It’s as though matted down grass in a field explains that an animal slept there without seeing the animal. Today, most astronomers accept that Dark Matter explains the unexpected motion in galaxies.

Nearly all astronomers agree that the universe has been expanding since the beginning of time, the Big Bang. Remember, mass attracts mass. The Earth attracts you and me, as evidenced by a bathroom scale. Expanding material in the universe is pulled back on itself, thus slowing the expansion. If the velocity from the Big Bang is low, the expansion would eventually stop, and a collapse would occur, becoming the Big Crunch. If the initial expansion rate is high, the expansion would slow, but continue forever.

In 1998, scientists measuring the rate of the universe’s expansion made a startling discovery. The expansion is apparently accelerating which Newton’s Laws can’t explain. For this discovery, three scientists were awarded the 2011 Nobel Prize in Physics. An accelerating expansion required explanation beyond the known laws of physics. One such idea, and currently the most popular, is the existence of an unknown energy or force. Playing on the popular term Dark Matter, the cosmologist Michael Turner in 1998 coined the term Dark Energy for this force. In this case, dark doesn’t refer to something unseen. Instead, it refers to something unknown. Like Dark Matter, scientists have been unable to identify Dark Energy. From Einstein’s famous equation, E=mc^2, we know that matter and energy are related. Using this equation and current measured parameters, NASA predicts that only about 5% of the universe is ordinary matter. More is Dark Matter, and most is Dark Energy. In other words, we don’t know what 95% of the universe is.

Science is not, as the expression goes, an exact science. Science is a method for finding physical truth. It is not truth itself. Science matures, evolves, and closes in on the truth. Over time, science increases the confidence in a theory. It never absolutely proves a theory. Scientists occasionally abandon previously accepted theories. Fifty years ago, in college, I wrote a paper comparing the Big Bang with a competing theory, the Steady-State Model of the universe. Today, scientists have all but abandoned the Steady-State Model. Over time, the theories of Dark Matter and Dark Energy will be either better understood, or abandoned.